1. Field of the Invention
The present invention relates to a joint compound for use in filling and coating the joints between adjacent gypsum wallboards, and more particularly refers to a joint compound of the type described which is of light weight, is readily sandable, has low shrinkage, provides a reduction in transportation costs because of its light weight, and additionally provides improved paintability properties over prior art lightweight joint compounds.
In the construction of buildings, one of the most common elements is gypsum wallboard, often known as "drywall", used to construct walls and/or ceilings. Walls made from gypsum wallboard are conventionally constructed by affixing the wallboard panels to studs or joists, and filling and coating the joints between panels with a specially prepared adhesive called a "joint compound". This process generally proceeds in the following fashion: a taping grade joint compound is placed within the joint formed by the abutted edges of the wallboards, and a liquid-permeable tape is embedded within the taping compound. When dry (or set), a second coating comprising a topping grade joint compound is applied over the joint. This may be sanded lightly, and then a third coat applied and conventionally finished. Another grade of joint compound which may be used is an all-purpose grade, as the name suggests. It is used both for embedding the tape and for applying the compound as a finishing coat. In some instances, a patterned effect or textured finish may be given to the finished wall and joint with the all-purpose joint compound.
The primary difference in the past between the various grades of joint compounds has been in the amounts of each ingredient utilized. Thus, no matter what the grade, joint compounds invariably include a filler and a binder. More binder is used for the taping grade than for the topping grade. Typical fillers may be calcium carbonate, calcium sulfate hemihydrate, or calcium sulfate dihydrate. When calcium sulfate hemihydrate is used, a setting-type joint compound is formed. An Example of a setting-type joint compound is disclosed in U.S. Pat. No. 3,297,601. When calcium carbonate or calcium sulfate dihydrate is utilized as a filler, the joint compound is a drying-type and must utilize a binder such as polyvinyl acetate to provide cohesion among the particles and adhesion to the wallboard paper.
In U.S. Pat. No. Re. 29,753, joint compounds are disclosed which are free of asbestos and utilize attapulgus clay to provide the non-leveling properties previously obtained by the use of asbestos fibers. Conventional compounds with or without attapulgus clay have proven to be excellent. However, these joint compounds are quite heavy and expensive to transport and heavy for the workmen to carry. Additionally, when conventional joint compounds dry, they acquire a hard texture and are difficult to sand in order to provide the joint with a smooth surface. It would be desirable to have a joint compound which is of lighter weight than conventional joint compounds, shrinks less, and which, when dry, can be readily sanded to provide a smooth surface.
In U.S. Pat. No. 4,454,267 a novel joint compound is disclosed and claimed comprising expanded perlite treated with a silicone polymer as one of the components to cause a substantial reduction in the density of the joint compound, with the resulting reduction in the cost of transportation and ease of working with the material. The joint compound disclosed in this patent was found to be very well suited for use in construction. However, it was found that, in part because of the mean particle size of the treated perlite, there resulted a considerable gloss difference between paint applied over the joint compound and paint applied over the wallboard paper itself.
In U.S. Ser. No. 752,268 filed July 5, 1985 and now U.S. Pat. No. 4,657,594 there is a disclosure of a joint compound utilizing expanded perlite which is treated with an amino-functional siloxane or silicone polymer together with a silicone wetting agent to improve the uniformity between the painted surface of a joint compound and the painted surface of the paper cover sheets of the gypsum wallboard. This resulted in an improvement in paintability as well as in adhesion of the joint compound to wallboard surfaces. However, the improvement was not as great as would be desired.